1. Field of the Invention
The present invention relates to a fiber grating type filter including an optical fiber in which a slanted grating is formed across a predetermined region along a longitudinal direction thereof, and to an optical fiber that can be applied to the fiber grating type filter.
2. Related Background Art
An optical component, obtained by forming a grating in the core region, in a predetermined region along a longitudinal direction of the optical fiber, is generally called as a fiber grating type fiber. There are two types of grating provided in thus filters; a Long-Period Grating (LPG) with a grating period of several hundred μm, and a Short-Period Grating (SPG) with a grating period of several hundred nm. Furthermore, a short-period grating can be classed as a Fiber Bragg Grating (FBG) whose grating plane is formed perpendicularly to the optical axis, and a Slanted Fiber Grating (SFG) whose grating plane is inclined by a predetermined angle with respect to the optical axis.
In the case that a loss filter based on an optical fiber of the kind described above is applied to a gain equalization for an optical amplifier, are turned reflection light with a large power will cause deterioration of the properties of the optical amplifier. Of the three types of grating described above, two types of grating, that is, the LPG and SFG type gratings are capable of reducing the power of the returned reflection light, and can be applied to loss filters for gain equalization. On the other hand, when equalizing gain of the optical amplifier, loss wavelength characteristics which can cancel wavelength dependence in the amplification gain are required. The SFG type filter has an advantage in that it allows desired loss characteristics to be created in a shorter length than the LPG type filter, and hence it is suitable in the loss filters for gain equalization.
Furthermore, in order to obtain desired loss characteristics as a gain equalizing filter in an optical amplifier, the SFG type filter is also required to have a narrow loss bandwidth. In the SFG, the angle of slant of the grating plane with respect to the optical axis is set to a predetermined value, in order to reduce the power of the returned reflection light. This angle of slant depends on the actual structure of the optical fiber in which the grating is to be formed. Generally, the increase of the angle of slant results the reduction of reflected light. On the other hand, in the SFG, the angle of slant must also be reduced in order to restrict the loss bandwidth. Consequently, there is a trade-off between reduction of the returned reflection light and compression of the bandwidth.
U.S. Pat. Nos. 6,292,606B1, 6,314,221B1 and 6,321,008B1 respectively disclose technology for resolving the aforementioned problem. An SFG type filter as disclosed in these patent publications is provided with an optical fiber comprising an inner core, an outer core, and a cladding region having a lower refractive index than the outer core, disposed so as to extending along a predetermined axis, and a slanted grating formed inside this optical fiber. In such an optical fiber, the inner core is composed of silica glass doped with P2O5 or Al2O3 as a refractive index increaser, and has no ultraviolet light photosensitivity. The outer core is composed of silica glass doped with GeO2 as a refractive index increaser, and does have ultraviolet light photosensitivity. Moreover, the cladding region is also comprised of silica glass, but this cladding region is doped with GeO2 in order that it is photosensitive to ultraviolet light. On the other hand, the cladding region is also doped with fluorine which serves to reduce the refractive index in such a manner that it is lower than that of either the inner core or the outer core.
In other words, in the optical fiber applied to the SFG type filter as disclosed in the foregoing patent publications, the core region with a high refractive index has a double-layer structure (dual core structure) comprising an inner core and outer core, and the cladding region has a structure of lower refractive index than the core region. Moreover, in the optical fiber having thus structure, while the outer part of the core region (outer core) and the cladding region are photosensitive to ultraviolet light, the inner part of the core region (inner core) is not photosensitive to ultraviolet light.
In this specification, “photosensitivity” means that the glass property change, for instance, the change of refractive index, when irradiated with light of a predetermined wavelength. For example, when silica glass that is photosensitive to ultraviolet light is irradiated with light of 270 nm or less (ultraviolet light), then the refractive index of the glass changes due to the reaction induced by ultraviolet radiation. Possible light sources for ultraviolet radiation include, for example, a KrF excimer laser (248 nm), a fourth-harmonic source (265 nm) of YAG laser, a second-harmonic source (244 nm) of Ar ion laser, or a second-harmonic source (255 nm) of Cu+ laser, or the like. When the optical fiber having a structure such as that described above is exposed in a ultraviolet intensity modulation which should be formed by means of a phase grating, intensity modulation mask, or the like, then a grating having a refractive index which varies periodically along the longitudinal direction of the optical fiber will be formed in the photosensitive glass region.
Therefore, in the SFG type filter as described in the aforementioned patent publications, no grating is formed in the inner core which has no photosensitivity, but a grating is formed in the outer core and cladding region which do have photosensitivity. In the SFG type filter having thus composition, both of the characteristics demanded for filter operation, namely, reduced the power of the returned reflection light and narrower bandwidth, are satisfied. Furthermore, since the angle of slant can be made relatively small, a merit is obtained in that the polarization dependent loss (PDL) that arises in accordance with the angle of slant is low.